The present invention relates to internal combustion engines of automotive vehicles and, particularly, to a gasoline-powered automotive internal combustion engine of the type using a catalytic converter in the exhaust system thereof for exhaust cleaning purposes. More particularly, the present invention is concerned with a method of improving the performance of an internal combustion engine of the specific type or the exhaust cleaning performance of the catalytic converter of the engine during cranking at low temperatures and further with an internal combustion engine adapted to put such a method into practice.
Some modernized automotive vehicles are equipped with catalytic converters in the exhaust systems of the engines for the purpose of converting toxic air contaminants in the exhaust emissions into harmless compounds before the emissions are discharged to the open air. A typical example of such catalytic converters is the one using an oxidative catalyst which is especially effective to re-oxidize the unburned combustile compounds such as hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gases emitted from the engine cylinders. The oxidative catalyst is not only reactive to these combustible compounds but is operable to reduce nitric oxides (N0.sub.x) in the exhaust gases if the exhaust gases to be processed by the catalyst are conditioned to have a chemical composition within a certain range which is dictated by the air-to-fuel ratio of the mixture supplied to the engine cylinders. The catalytic converter using the oxidative catalyst thus exhibits tripple effects to the exhaust gases of an internal combustion engine and is capable of reducing the most important air contaminative sources of engine exhaust emissions in a single unit when the combustible mixture to be supplied to the engine cylinders is proportioned to have an air-to-fuel ratio within the certain range. Experiments have revealed that it is the stoichiometric air-to-fuel ratio of about 14.8:1 that enables the tripple-effect or "three-way" catalytic converter to produce its maximum conversion efficiency against the three kinds of air contaminative compounds. It is, for this reason, desirable to provide in an internal combustion engine using a tripple-effect catalytic converter suitable mixture control means adapted to regulate the air-to-fuel ratio of the mixture produced in the mixture supply system of the engine toward the stoichiometric level or maintain the air-to-fuel ratio of the mixture within a predetermined range containing the stoichiometric value.
The mixture control means thus used in combination with a tripple-effect catalytic converter was initially of a so-called "open-loop" type which operates without respect to the conditions of the exhaust gases that vary in composition and temperature depending upon the operational and ambient conditions of the engine. Difficulties were encountered in accurately controlling the air-to-fuel ratio by the use of such mixture control means because the density and viscosity of the fuel delivered to the mixture supply system of the engine are subject to fluctuation due to the fluctuations in the pressure and temperature of atmospheric air, the temperature of the fuel fed to the mixture supply system and other operational and ambient conditions of the engine. The fluctuations in the air-to-fuel ratio of the mixture result in fluctuations in the concentrations of the air contaminative compounds in the exhaust gases emitted from the engine cylinders.
To provide a solution to the problems arising from the use of mixture control means of the open-loop type, a mixture control system of a "closed-loop" or "feedback" type has been proposed which is adapted to control the air-to-fuel ratio of the mixture on the basis of information fed back from the exhaust system.
A mixture control system of the closed-loop or feedback type involves an exhaust sensor operative to detect the concentration of a prescribed type of chemical component contained in the exhaust gases from the engine cylinders and produce an analog output signal, usually in the form of voltage, indicative of the detected concentration of the particular chemical component in the exhaust gases. The chemical composition of exhaust gases is a faithful representation of the air-to-fuel ratio of the mixture delivered to the engine cylinders and, for this reason, the mixture control system of the closed-loop or feedback type is capable of accurately and constantly monitoring the actual air-to-fuel ratio of the mixture produced in the mixture supply system of the engine. The air-to-fuel ratio of the mixture supplied to the engine cylinders is thus accurately regulated at all times toward the stoichiometric level or maintained within a certain narrow range containing the stoichiometric level irrespective of the fluctuations in the various operational and ambient conditions of the engine. As the chemical component of the exhaust gases to be detected, any one or more of oxygen, carbon monoxide, carbon dioxide, hydrocarbons and nitric oxides may be selected although oxygen is preferred for ease of detection. The catalytic converter has been assumed to be of the tripple-effect type in the foregoing description but the essential features of the mixture control system of the closed-loop or feedback type can be exploited not only in an internal combustion engine arranged with a tripple-effect catalytic converter but when combined, in effect, with another type of catalytic converter reactive to one or two of the above-mentioned three types of air contaminative compounds if the mixture control system is designed and arranged in such a manner as to regulate the air-to-fuel ratio of the mixture toward a predetermined value which is optimum for the particular function of the converter or maintain the air-to-fuel ratio of the mixture within a certain range containing such a predetermined value.
By virtue of the mixture control means of the closed-loop or feedback type, the catalytic converter in the exhaust system of a gasoline-powered internal combustion engine is enabled to produce its maximum exhaust cleaning performance under various modes of operation of the engine because the air-fuel mixture supplied to the engine cylinders is at all times proportioned to have an air-to-fuel ratio optimum for the intrinsic function of the catalytic converter. As will be readily understood by those skilled in the art, however, controlling the air-to-fuel ratio of combustible mixture toward a fixed value throughout the varying operating conditions of the engine create problems during some modes of operation of the engine. During cranking of the engine at low temperatures, for example, it is desirable to have more than normal amounts of fuel delivered to the air stream through the mixture supply system so as to enrich the air-fuel mixture to be produced in the mixture supply system and to enable the fuel to readily evaporate in the system, as is well known in the art. If the mixture is produced under the control of the feedback mixture control system during such a condition of the engine, the mixture could not be made richer than the mixture having the air-to-fuel ratio provided by the control system and, as a consequence, will cause the engine to fail to properly operate.
The exhaust sensor incorporated into the mixture control system of the above described nature is usually composed of an electrolytic element of sintered zirconium oxide coated with microporous layers of platinum if oxygen is selected as the chemical component whose concentration in the exhaust gases is to be detected. The electrolytic element is oxygen ion conductive at temperatures within a certain range of, for example, 400.degree. C. to 900.degree. C. and produces between the layers of platinum a voltage that varies with the difference between the partial pressures of oxygen to which the platinum layers are exposed, viz., the difference between the concentration of oxygen in the exhaust gases and the concentration of oxygen in the atmospheric air. When, therefore, the temperature of the exhaust gases is lower than such a range during cranking of the engine, the exhaust sensor is disabled from producing a reliable output signal which faithfully indicates the concentration of the oxygen content in the exhaust gases passed through the exhaust sensor and, as a consequence, the reliability of the mixture control system as a whole is critically impaired. This, in turn, results in deterioration of the performance efficiency of the catalytic converter and, coupled with the fact that the engine tends to emit more than normal quantities of hydrocarbons and carbon monoxide during cranking, gives rise to an increase in the total concentration of the air contaminative compounds in the exhaust gases discharged to the open air. These problems are also encountered more or less in an internal combustion engine arranged with a mixture control system using another type of exhaust sensor reactive to carbon monoxide or dioxide, hydrocarbons or nitric oxides in the exhaust gases.